Sclerotinia infects over 400 species of plants throughout Canada, including numerous economically important crops such as Brassica species, sunflowers, dry beans, field peas, lentils, and potatoes (Boland and Hall (1994) Can. J. Plant Pathol. 16:93-108). Sclerotinia sclerotiorum is responsible for over 99% of the disease, while Sclerotinia minor produces less than 1% of the disease. Sclerotinia produces sclerotia, which are irregularly shaped dark overwintering bodies that can endure in soil for four to five years. The sclerotia can germinate carpogenically or myceliogenically depending on the environmental conditions and crop canopies. The two types of germination cause two distinct types of diseases. Sclerotia that germinate carpogenically produce apothecia and ascospores that infect above-ground tissues, resulting in stem blight, stalk rot, head rot, pod rot, white mold and blossom blight of plants. Sclerotia that germinate myceliogenically produce mycelia that can infect root tissues, causing crown rot, root rot and basal stalk rot.
Sclerotinia causes Sclerotinia stem rot, also known as white mold, in Brassica, including canola. Canola is a type of Brassica having a low level of glucosinolates and erucic acid in the seed. The sclerotia germinate carpogenically in the summer, producing apothecia. The apothecia release wind-borne ascospores that travel up to one kilometer. The disease is favored by moist soil conditions (at least 10 days at or near field capacity) and temperatures of 15-25° C., prior to and during canola flowering. The spores cannot infect leaves and stems directly. They must first land on flowers, fallen petals, and pollen on the stems and leaves. Petal age affects the efficiency of infection, with older petals better able to effect infection (Heran et al. (1999) “The Effect of Petal Characteristics, Inoculum Density and Environmental Factors on Infection of Oilseed Rape by Sclerotinia sclerotiorum” The Regional Institute Ltd. http://www.regional.org.au/au/gcirc/3/428.htm). The fungal spores use the flower parts as a food source to germinate and infect the plant.
Brassica can also develop root rot under certain conditions. For example, winter and spring canola occasionally develop root rot during mild winters in Europe (winter canola) and in Georgia, US (spring canola).
The severity of Sclerotinia in Brassica is variable, and is dependent on the time of infection and climatic conditions (Heran et al., supra). The disease is favored by cool temperatures and prolonged periods of precipitation. Temperatures between 20 and 25° C. and relative humidities of greater than 80% are required for optimal plant infection (Heran et al., supra). Losses ranging from 5 to 100% have been reported for individual fields (Manitoba Agriculture, Food and Rural Initiatives, 2004). On average, yield losses equal 0.4 to 0.5 times the percentage infection. For example, if a field has 20% infection (20/100 infected plants), then the yield loss would be about 10%. Further, Sclerotinia can cause heavy losses in wet swaths.
The symptoms of Sclerotinia infection usually develop several weeks after flowering begins. The plants develop pale-grey to white lesions, at or above the soil line and on upper branches and pods. The infections often develop where the leaf and the stem join because the infected petals lodge there. Infected stems appear bleached and tend to shred. Hard black fungal sclerotia develop within the infected stems, branches, or pods. Plants infected at flowering produce little or no seed. Plants with girdled stems wilt and ripen prematurely. Severely infected crops frequently lodge, shatter at swathing, and make swathing more time consuming. Infections can occur in all above ground plant parts especially in dense or lodged stands. Once plants are infected, the mold continues to grow into the stem and invade healthy tissue. New sclerotia are formed to carry the disease over to the next season.
Some varieties of canola with certain morphological traits are better able to withstand Sclerotinia infection. For example, Polish varieties (Brassica rapa) have lighter canopies and seem to have much lower infection levels. In addition, petal-less varieties (apetalous varieties) do not provide the initial infection source (i.e., the flower petal) and avoid Sclerotinia infection to a greater extent (Okuyama et al. (1995) Bulletin of the Tohoku National Agricultural Experiment Station. National Agriculture Research Center, Tsukuba, Ibaraki 305, JAP 3-1-1an. 89: 11-20; Fu (1990) Acta Agriculture Shanghai. Economic Crop Research Institute, Jiangsu Province Academy of Agricultural Sciences, Nanjing 210024, China 6 (3): 76-77. Other examples of morphological traits that confer a degree of reduced susceptibility to Sclerotinia in Brassica include increased standability, lower petal retention, higher branching (both extent and position), flowering (early start and/or short duration) and early leaf abscission. Jurke and Fernando (“Plant Morphology of Canola and its Effects on Sclerotinia sclerotiorum infection in ICPP” 2003 8th International Congress of Plant Pathology, New Zealand) screened eleven canola genotypes for Sclerotinia disease incidence. Significant variation in disease incidence was explained by plant morphology and the difference in petal retention was identified as the most important factor. However, these morphological traits alone do not confer resistance to Sclerotinia and most canola lines in Canada are considered susceptible to Sclerotinia. 
The primary means of controlling Sclerotinia in infected canola crops is by spraying with fungicide. Typical fungicides used for controlling Sclerotinia on Brassica include Rovral™/Proline from Bayer and Ronilan™/Lance™ from BASF. If infection is already evident, there is no use in applying fungicide as it is too late to have an effect. Accordingly, growers must assess their fields for disease risk to decide whether to apply a fungicide. This can be done by using a government provided checklist or by using a petal testing kit. Either way, the method is cumbersome and prone to errors.
Numerous efforts have been made to develop Sclerotinia resistant spring Brassica plants. Built in resistance would be more convenient, economical, and environmentally friendly compared to controlling Sclerotinia by application of fungicides. Since the trait is polygenic it would be stable and not prone to changes in efficacy, as fungicides may be. Winter canola is also susceptible to Sclerotinia. 
Spring canola (Brassica napus subsp. oleifera var. annua) differs from winter canola (Brassica napus subsp. oleifera var. biennis) primarily in the absence of an obligate vernalization requirement. Asiatic rapeseed and canola versions have a low to intermediate requirement for vernalization. While winter canola cannot finish its reproduction cycle when planted in the spring, Asiatic material cannot finish its reproduction cycle if planted in late spring, but early spring planting and exposure to cold enables Asiatic material to flower and set seed. In controlled conditions winter material requires 12-14 weeks of vernalization while Asiatic material requires 2-8 weeks. Table 1 summarizes the differences between winter, semi-winter (Asiatic) and spring canola varieties.
TABLE 1Main determinations of growth habit in Brassica napus materialsSemiTypeSpring*SpringWinterWinterGrowingCanada,AustraliaChina, JapanEuropeareasEuropeVernalizationNoneNone2-8 weeks12-14 weeksRequirementIntermediatestrong orfullTime ofSpringFallFallFallseeding(Increasing(Decreasing(Decreasing(DecreasingDay Length)Day Length)Day Length)Day Length)Number of30-9090-150120-180150-270days untilflowering*Canadian, European and Australian spring materials can be planted and grown in any environment or seeding time for spring canola.
Some Chinese cultivars of rapeseed/canola are partially resistant to Sclerotinia. For example, ChunYun et al. ((2003) Acta Agronomica Sinica 29 (5): 715-718); HanZhong et al. ((2004) Scientia Agricultura Sinica 37 (1): 23-28); WeiXin et al. ((2002) Chinese Journal of Oil Crop Sciences 24 (3): 47-49); YongJu et al. ((2000) Chinese Journal of Oil Crop Sciences 22 (4): 1-5) describe partially resistant varieties of rapeseed. However, some of these varieties are not canola quality and all of them require vernalization. The partial field resistance in Chinese varieties originated from the rapeseed variety Zhong you 821. Despite improvements in partial resistance in Zhong you 821, its reaction to pathogens is less stable under environmental conditions favorable for development of Sclerotinia (Li et al. (1999) “Breeding, inheritance, and biochemical studies on Brassica napus cv. Zhongyou 821: Tolerance to Sclerotinia sclerotiorum (stem rot)”. Proceedings of the 10th International Rapeseed Congress, Canberra, Australia).
Some Japanese cultivars of rapeseed have partial stem resistance to Sclerotinia. Partial stem resistance was detected by indoor tests in comparison with winter canola (Brun et al. (1987) “A field study of rapeseed (Brassica napus) resistance to Sclerotinia sclerotiorum.” 7th International Rapeseed Congress, Poznan, Poland). However, these varieties are not canola quality and are semi-winter types (see Table 1).
Breeding for Sclerotinia resistance in canola has been very difficult due to the quantitative nature of this trait. Further, the incorporation of physiological resistance with morphological traits that avoid or reduce infection multiplies the complexity of breeding for resistance. In addition, it has been very difficult to screen for resistance because of the direct environment by genetic (GXE) interaction (i.e., temperature and humidity requirements, as well as microenvironment requirements) with the plant. As stated above, there are few Canadian spring Brassica varieties with resistance to Sclerotinia, this despite many years of co-evolution and environmental pressure to select for this trait. A level of field resistance in rapeseed (and recently some canola materials) was attained via breeding efforts in China as described with Zhong you 821 (Li et al., supra). However, the levels of such partial resistance or tolerance are relatively low and fungicide applications are still recommended on all rapeseed and canola materials in China (verbal communication) (Hu et al. (1999) “Effect of cultural control on rapeseed stem rot (Sclerotinia sclerotiorum) in Brassica napus.” Proceedings of the 10th International Rapeseed Congress, Canberra, Australia). Other breeding efforts included quantitative trait loci analysis (Zhao and Meng (2003) Theoretical and Applied Genetics 106 (4): 759-764), mutagenesis breeding (Mullins et al. (1999) European Journal of Plant Pathology 105 (5): 465-475; Wu et al. (1996) Sichuan Daxue Xuebao (Ziran Kexueban) 33 (2): 201-205; LiangHong et al., 2003, extensive screening efforts (Sedun et al. (1989) Canadian Journal of Plant Science 69 (1): 229-232; Zhao et al. (2004) Plant Disease 88 (9): 1033-1039); and screening for expressed sequence tags (ESTs) (Li et al. (2004) Fungal Genetics and Biology 41 (8): 735-753) to name a few. Several spring canola varieties with moderate tolerance to Sclerotinia have been developed (Ahmadi et al. (2000) Seed and Plant 16 (1): Pe127-Pe129, en14; Ahmadi et al. (2000) Introduction of rapeseed (Brassica napus L.), cultivar Esteghlal. Seed and Plant 16 (1): Pe127-Pe126, en13; BaoMing et al. (1999) Chinese Journal of Oil Crop Sciences 4: 12-14; and Liu et al. (1991) Scientia Agricultura Sinica 24 (3): 43-49), however the level of tolerance is low and the lines cannot withstand high disease pressure. Recently, transgenic canola has been developed carrying an oxalic oxidase gene (U.S. Pat. No. 6,166,291 and divisional patents thereof) however there are regulatory and social problems associated with transgenic plants. Accordingly, significant technical human intervention is required to breed canola varieties that are resistant to Sclerotinia. 
More recently, Brassica and canola varieties with high levels of resistance to Sclerotinia were developed after a long and intensive breeding program (See, for example, WO 2006/135717, the entire teachings of which are hereby incorporated by reference). This approach is very time and labor intensive, and requires a long time to determine whether the breeding program is successful. The difficulty in breeding for whole plant field resistance to Sclerotinia is due, at least in part, to the multigenic nature of this trait.
What is needed in the art and industry is a means to identify genes conferring whole plant field resistance to Sclerotinia, using molecular markers. These markers can then be used to tag the favorable alleles of these genes in segregating populations and then employed to make selection for resistance more effective. The present invention provides this and other advantages.